This invention relates to a variable gain controller, and more particularly to a variable gain controller utilized for noise reduction systems.
In the past, there has been proposed a gain controller for a noise reduction system such as the Dolby-B noise reduction system which is used to reduce the level of background noise introduced during recording and playback of audio signals on magnetic tape, and to improve the noise level in FM broadcast reception.
The Dolby B-type noise reduction system contains a high-pass dynamic filter circuit comprising a variable resistor which controls the response of the filter and operates to boost low-level high-frequency signals in the encode mode and to attenuate the same signals in a complementary manner in the decode mode.
Consequently, it is said that frequency-gain response (hereinafter called gain response) of the filter can be varied to some extent by means of the variable resistor.
However, the filter circuit is originally designed to pass the low frequency signals, and it does not function as high-pass filter or band-pass filter.